The present invention relates to the preparation of explosive compositions by combining a blend of a water-in-oil emulsion and solid particulate inorganic nitrate in the form of prills or granules. More particularly, the present invention relates to a method for stabilizing explosive compositions by cooling the emulsion in a shell and tube heat exchanger. The invention also concerns the fabrication of storage-stable explosive compositions useful in the loading and detonating of drill holes.
Explosives which comprise a blend of a water-in-oil emulsion and solid particulate inorganic nitrate, such as ammonium nitrate (AN), have always captured the interest of blasters owing to the fact that they are able to offer the advantages of high bulk density, blasting energy, and water resistance characteristic of emulsion explosives, while at the same time resulting in cost reductions owing to the lower cost of AN. Among the problems that may be encountered in connection with the use of emulsion explosives, however, are those of emulsion stability during processing, and the stability of the blend's explosive and theological properties.
An inherent problem with emulsion explosives is their relative instability, due to the fact that they comprise a thermodynamically unstable dispersion of supercooled solution or melt droplets in an oil-continuous phase. If the emulsion remains stable during processing, the supercooled droplets are prevented from crystallizing or solidifying into a lower energy state. If the emulsion weakens or becomes unstable, however, then crystallization or solidification of the droplets results, and the explosive generally loses at least some of its sensitivity to detonation and becomes too viscous to handle for certain blasting applications.
Moreover, when solid components are added to emulsion explosive, such as glass microspheres for density reduction or AN/ANFO prills for increased energy or particles of oxidizer salt, such solid components tend to destabilize emulsions even further. The solid components may disrupt the continuous fuel phase and provide a site for resulting crystallization of the discontinuous oxidizer salt solution phase. In addition, the prills often contain fines and/or clay or have a coating that act as poisons to the emulsion thereby hastening its destabilization. Since emulsion and prill combinations must remain stable during handling and for a period of time after being loaded into a drill hole in order to remain reliably detonable, the presence of AN or ANFO prills can present serious stability problems.
There have been processes developed in the industry for purposes of increasing a blend's explosive and theological properties. In U.S. Pat. No. 3,642,547 (Conrad) an emulsion explosive is stabilized during sensitization by homogeneously mixing a gas into an emulsion. This controls the product density and allows storage of the explosive for over a year without affecting the detonation sensitivity. U.S. Pat. No. 5,076,867 (McKenzie), relates to a method of stabilizing a mixture of emulsion and AN or ANFO prills by dissolving a surfactant in a liquid organic fuel prior to adding the fuel to the AN prills. Additionally, efforts have been made to reduce water loss from the emulsion blend to prevent crystallization and subsequent desensitization of the blend. See U.S. Pat. No. 4,555,278 (Cescon et al.).
Safety is another concern in the industry. There have been several methods developed in the explosives industry to increase the safety of manufacturing emulsion explosives. For example, U.S. Pat. No. 3,766,820 describes a process of detecting and trapping possible detonations of melted explosives.
U.S. Pat. No. 5,076,867 relates to a method for stabilizing a detonable mixture of emulsion and AN or ANFO prills. The method involves dissolving a surfactant in a liquid organic fuel prior to adding the fuel to AN prills for forming ANFO prills, or if AN prills are used without added liquid organic fuel, the surfactant is added to the prills. The prills containing the surfactant are then mixed with the emulsion.
Other efforts have been directed to reducing agitation of sensitive emulsion explosives by developing improved mixing devices. In U.S. Pat. No. 4,213,712 (Aanonsen et al.), a rotor mixer which traps detonations of an emulsion explosive is described. Additionally, emulsion explosives have been mixed using a combination of an in-line static mixer and a low-shear mechanical mixer in order to reduce generation of heat during high-shear mechanical mixing. See U.S. Pat. No. 4,948,440 (Cribb et al.)
However, there remains a need for increased safety during the manufacture of explosive compositions. Moreover, there is also a need for an explosive emulsion having stable explosive and rheological properties.